1. Field of the Invention
The present invention relates to an automatic transmission control system for use in vehicles such as automobiles. More particularly, the invention relates to an automatic transmission control system for a transmission which has a speed change gear, a torque converter, and a lockup mechanism for the torque converter.
2. Description of the Related Art
Heretofore, the control of the lockup mechanism of a torque converter in the transmission of a vehicle has been performed by controlling the timings of lockup "ON" and lockup "OFF" on the basis of the speed of the vehicle and the opening of a throttle valve for the engine of the vehicle.
Although not directly pertinent to the controls of the timings of lockup "ON" and lockup "OFF", a technique which changes the engagement force of a lockup clutch in accordance with a running resistance is disclosed in the official gazette of Japanese Patent Application Laid-open No. 212668/1990. Besides, a technique in which a gear shift is effected in accordance with a running resistance or the like is disclosed in the official gazette of Japanese Patent Application Laid-open No. 112852/1986.
The prior-art lockup control stated above has disadvantages as exemplified below. When the vehicle is ascending a slope, the lockup of the lockup mechanism does not turn OFF easily, and the speed change gear of the transmission cannot be downshifted. Moreover, even when the speed change gear has been downshifted, when descending a slope, the lockup does not turn ON easily, and engine braking cannot be applied. That is, with the prior-art lockup control, the timings of the lockup are controlled depending upon only the vehicle speed and the throttle valve opening. Therefore, even in a case where the running resistance of the vehicle has increased in, e.g., the ascent of the slope, the lockup proceeds basically at the same timing as in the running of the vehicle on level ground. This poses the problem that the drivability of the vehicle is not very good.